The invention relates to automatic focusing systems for microscopes, especially interference microscopes, and more particularly to automatic focusing systems which directly detect the presence of interference fringes to determine the degree of focus of the optical system.
In a typical interference microscope, light from a source, typically a laser, is split by appropriate means to travel down a reference path and a sample path. The reference path and sample path differ in that the reference path is focused on a reflective reference mirror and the sample path is focused on the sample to be measured. Light reflected from the reference mirror and light reflected from the sample interfere to create an interference pattern which is detected by a photodetector array. The resulting signals are analyzed using various well known interferometric techniques to determine the topography of the sample surface. Accurate data can be obtained from an interference microscope only if the "focus error" is minimal.
All presently known interference microscopes are manually "randomly" focused by a human operator who must adjust the axial position of the interference microscope objective until a sample surface visually appears to be "in focus". Unfortunately, the interference pattern is only present over a narrow axial range, typically a few microns, near the "ideal" focus range of the microscope objective relative to the sample surface. This makes it difficult to focus an interference microscope.
This narrow axial range where fringes are visible is very difficult for a human operator to detect as he or she "adjusts" the microscope objective through the focus range of the microscope, due to limitations in the sampling rate of the average human eye. Furthermore, human reflex rate limitations limit the ability to stop movement of the microscope objective once visual observation of interference fringes has occurred. This is especially true for low power microscope objectives. For low power microscope objectives, the depth of focus is so large that images which appear to the human eye to be well focused in fact may not be accurately focused. That is, when considering an interference microscope for lower magnifications, the test surface may appear to be in focus to the eye of the observer over a fairly large range of movement of the microscope objective, but the fringe pattern will be visible only within an axial range of a few microns.
It sometimes requires a human operator a long time (e.g., many minutes) to achieve accurate focusing of a typical interference microscope being utilized to observe the surface of a typical sample. After the interference microscope is finally focused, then disturbing it, for example to insert or remove a filter, or to cause lateral movement of an X-Y stage to observe an adjacent area of the sample surface often results in loss of focus. The operator therefore may need to refocus each observed area of the sample. It is not an uncommon occurrence for a manual focusing adjustment by a relatively inexperienced operator to result in "crashing" the microscope objective into the sample surface, possibly destroying both.
Many automatic focusing devices for various optical systems are known. Most, such as the automatic focusing devices disclosed in Pat. Nos. 4,600,832, 4,333,007, 4,385,839, 3,798,449, and 4,207,461, require that discernable features of the sample be present. The contrast of the images of such features is utilized to accomplish automatic focusing. Some of these references utilize auxiliary optical sources and additional apertures to obtain focus information. One reference, Pat. No. 4,620,089, uses an interference pattern that is present when the object is in focus. This system requires differential sensing of the interference pattern by means of two optical paths and two detectors.
Another prior art technique involves sensing focus error. Focus error sensing techniques usually require using a laser as the light source.
There clearly is an unmet need for an automatic focusing apparatus for an optical system, especially for an interference microscope, that avoids the large amount of time often required for manual focusing of interference microscopes. There also is an unmet need for an automatic focusing system, especially for an interference microscope, which can accurately and repeatedly focus the optical system or interference microscope to thereby eliminate human focusing errors which are inherent in present manual focusing techniques.